Melt-spun elastomeric fibers have been in the past unacceptable for use as textile fibers because they have very low tenacity and poor recovery from elongation. Also, melt-spinnable thermoplastic urethane polymers have in the past formed gels too readily, thus inhibiting spinning of the material.
Prior art methods of forming melt-spun polyurethane fibers include the use of a “one shot process” wherein mixtures of diisocyanates, a polymeric glycol “soft” segment, and a low molecular weight glycol and a catalyst are all added to an extruder with mixing at a temperature of about 100° C. to about 250° C. and a time of about 1–5 minutes to obtain, after spinning, a polyurethane fiber. The properties of this fiber are limited because of the randomness of the polymerization process, the required rapid reaction and the substantially poor mixing. The urethane groups in the final product have no orderly arrangement, causing poor tenacity, unacceptable recovery from elongation and an unacceptable amount of gel formation.
The preparation of plastics in extruders is generally known. In contrast to the processing of plastics by a purely thermoplastic method, the term “reaction extrusion” is used in the case of such polymer synthesis in an extruder. Accordingly, the extruder used as the chemical reactor is frequently also referred to as a “reaction extruder”.
Almost without exception, the literature recommends the twin-screw extruder in which both screws rotate in the same direction, for the synthesis of polyurethanes. Single-screw extruders and twin-screw extruders having counter-rotating screws are unsatisfactory owing to the poor mixing effect during passage through the extruder, and extruders having more than two screws are too expensive.
U.S. Pat. No. 5,621,024 (Eberhardt, et al) discloses a process for preparing thermoplastic polyurethane employing a twin-screw extruder having multiple zones. All reactants are fed as a low viscosity mixture to the extruder in a “one shot” process.
U.S. Pat. No. 5,136,010 (Reisch, et al) discloses a method of preparing a cast polyurethane or polyurea elastomer. The method comprises the steps of: fabricating a polyol having a specified molecular weight, reacting the polyol with a polyisocyanate to obtain an isocyanate-terminated prepolymer, and reacting the prepolymer with a non-(ethylene glycol) chain extender. The elastomer is prepared in a “one shot” process.
U.S. Pat. No. 5,116,931 (Reisch, et al) discloses a thermoset polyurethane or polyurea elastomer prepared in a “one shot” process.
U.S. Pat. No. 5,096,993 (Smith, et al) discloses a thermoplastic polyurethane or polyurea elastomer made by reacting in a “one shot” process a polyether diol, diisocyanate and a chain extender. The chain extender is a difunctional, isocyanate-reactive material.
U.S. Pat. No. 3,233,025 (Frye, et al) discloses a method of forming a thermoplastic polyurethane having free isocyanate groups. The method comprises the steps of mixing reactive components comprising an excess of an organic polyisocyanate and an organic compound containing at least two active hydrogen containing groups, reacting in a passageway at a temperature of about 60° C. to about 250° C. and a limited residence time, and removing the thermoplastic polyurethane. A twin-screw extruder can be employed.